Leaf type or blade type mechanical shutters have been used to regulate the amount of light that is going to reach the imaging areas of a filmstrip contained within a camera. Typically, the leaf type of shutter is an array of metal "blades" that are pivoted so that they all swing towards or away from the lens aperture of the camera. When the shutter is closed, all the blades are overlapping in the center of the lens aperture and no light reaches the filmstrip. To open the shutter, the blades pivot away from the center of the aperture, so that light may pass through the aperture and expose the filmstrip. Sometimes leaf type mechanical shutters may have one or more blades, each of which may have a different diameter opening. When a picture is taken the blade or blades swing away from the center of the lens aperture so that light may pass through the aperture, through the blade or blade openings to expose the filmstrip. If a picture is not being taken, the shutter will be closed. The power to open and close the leaf type mechanical shutters is provided by a spring or springs that are under tension and the timing is controlled by a watch-type gear train.
Electromagnetic shutters may be electronically controlled. The manufacturing cost of an electromagnetic shutter is less than the manufacturing cost of mechanical shutters.
The prior art utilized electromagnetics to control one or more shutter blades that were capable of having two or more aperture openings. Typically a magnet that was coupled to one or more shutter blade was placed between the poles of an electromagnet. When current in one direction was applied to the electromagnet, the shutter blades formed one aperture opening and when current was applied to the electromagnet in the opposite direction the shutter blades formed another aperture opening. If no current was applied to the electromagnet the magnet remained in its neutral position due to reluctance forces. The neutral position of the actuator was determined by the magnetic circuit of the electromagnet, or by some mechanical device that was attached to the actuator and supplied a restoring force to the actuator, such as a mechanical detent device. The restoring force would bring the actuator or pivot arm back to the neutral position.
If no device was used to bring the actuator to its neutral position the actuator would oscillate around its neutral position until it reached its equilibrium position. The oscillations were undesirable since the location of the actuator was not known at all times. Thus, in a totally magnetic circuit it took time for the actuator to settle down to its detent or balance point.
If a mechanical device e.g. a spring was used to bring the actuator to its balance point. The spring would have to be physically attached to the actuator and to a solid portion of the camera, with no intervening objects between the spring attachment point and the actuator. Thus, the use of a spring caused space and positioning problems. Another disadvantage in using a spring was that the physical characteristics of the spring would not remain constant as the spring aged. Thus, the same amount of restoring force would not be placed on the spring thereby impacting actuator performance.